Preparation of Tertiary Alkyl Chlorides by Nucleophilic Aliphatic
Substitution/Sodium Iodide and Silver Nitrate Tests for Alkyl Halides
April 13, 2007
Methods and Background
The purpose of the experiment was to demonstrate that a tertiary alcohol, 2-methyl-2-butanol,
reacts with hydrochloric acid through unimolecular nucleophilic aliphatic substitution to form a
tertiary alkyl chloride, 2-chloro-2-methylbutane; furthermore, sodium iodide and silver nitrate
tests were used to confirm the presence of a tertiary alkyl chloride.
Nucleophilic aliphatic substitution involves the substitution of one group for another at a
-hybridized carbon atom.
In nucleophilic aliphatic substitution, a nucleophile, a
neutral molecule or anion that has at least one nonbonding pair of electrons, and thus Lewis base
character, attacks an electrophilic, Lewis acidic, carbon atom, donating its nonbonding pair of
electrons to form a new covalent bond, while the leaving group, which is neutral or negatively
charged, departs as it accepts the pair of bonding electrons from the carbon atom as the bond
between it and the carbon atom breaks.
The reaction can be considered to be a Lewis acid-base
reaction because the electronegative leaving group imparts Lewis acidic character on the carbon
atom to which it is attached due to polarization of the bond.
One mechanism of nucleophilic aliphatic substitution is unimolecular nucleophilic substitution
In this mechanism, the bond between the carbon atom and the leaving group undergoes
heterolytic cleavage, or ionization, utilizing the assistance of the polar interactions between
solvent molecules and the incipient cationic and anionic centers, to yield a carbocation, an
intermediate with a positively charged carbon atom (the carbon atom that was bonded to the
The carbocation then combines with a nucleophile to form the substitution
The first step of an S
1 reaction, formation of a carbocation, is much slower than the second step
because it involves breaking the bond between the carbon atom and the leaving group to generate
an unstable carbocation, which is an endothermic process; therefore, it is the rate-determining
step of the reaction.
In contrast, the second step, nucleophilic attack of the carbocation to yield
the product, is much faster because it is an exothermic process.
Since the first step of the
reaction is the rate-determining step, the rate of the reaction depends only upon the concentration